Electric fuse



Oct. 22, 1935. v. HOPE 2,018,556

ELECTRIC Fuss Filed Oct. 4, 1933 3 Sheets-Sheet 1 Fig.1. Fig.2. Fig. 3. Fig.4. Fi .7. Fig.5.

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V. HOPE ELECTRIC FUSE Filed Oct. 4, 1933 Fig. [GA

Oct. 22, 1935. v. HOPE 2,018,556

ELECTRIC FUSE Filed Oct. 4, 1933 3 Sheets-Sheet 3 Fig. 1

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W Y5 #M QMJAAM F M Patented Oct. 22, 1935 UNITED STATES PATENT OFFICE Application October 4, 1933, Serial No. 692,205 In Great Britain October 13, 1932 11 Claims.

This invention relates to electric fuses of the enclosed powder-filled cartridge type and whilst applicable to this type generally, is more particularly applicable to electric fuses of the bi-metallic type, in which the fuse on short circuit blows" by the fusing of a metal having a relatively high fusing point, and upon moderate overloads blows by the melting of a metal having a comparatively low melting point.

As a result of a series of tests arranged to subject to the most exacting conditions, fuses having a fusible element comprising a low melting point metal such as tin, lead or alloys of these metals, it has been found that when a fuse rated to clear the circuit for an overload of 30% above normal, is subjected to a persistent overload of 25% to 28% for a long period, the low melting point metal can, without breaking the circuit, be raised to a temperature high enough to damage the fuse and render it inoperative; that is to say the low melting point metal when heated to its melting point temperature does. not "run" and break the circuit; and the object of the present invention is to overcome this difficulty, and, generally, to ensure definite and certain clearing action of an electric fuse under all the usual conditions met with in practice.

As a result of research involving a series of numerous practical tests extending over a considerable period and made with many different materials, I have found that the clearing action of a low melting point metal of a powder-filled enclosed fuse, and particularly in bi-metallic fuses, is improved in the most marked manner by the employment, as a clearing, agent, of boric acid, which has certain definite characteristics which will be more fully described hereinafter. The boric acid may be in the form of soap crystal B2O3.3H2O; or meta-boric acid, BzOa.Ha0; or pyro-boric acid 2(B2O3) .HzO; or boric-anhydride B203.

In one modification of the invention I apply, for example, by coating, to a fusible element of low melting point, a suitable quantity of one of the above forms of boric acid; by preference I employ a mixture of meta and pyro-boric acid, mainly because it can be used in plastic state and can in practice be applied easily and effectively.

Where the metal of low melting point is fltted in hollow form, I find it desirable in addition to coating the exterior of the tube or capsule of the said metal with plastic boric acid, also to charge the interior of the tube or capsule with boric acid, preferably in the form of boric-anhydride.

I further find when the fuse blows by the fusing of a metal of low melting point to which the boric acid had been applied, that not only does the fusible metal run" but that the severed ends of the fusible element are effectually sealed by the boric acid or .a combination of the latter and the filling powder, and are insulated so as to prevent the continuance or reestablishment of the circuit.

As a result of my experiments and observations it would appear that the effective and deflnite clearing described upon prolonged moderate overloads is due to the fact that the boric acid has the following ,characteristics:--It melts at a temperature at or about that at which the low melting point metal melts; it is not, when applied, 15

a conductor of electricity; it is not, when decomposed at the temperature of the electric arc, itself a conductor, nor does it form a product which is a conductor of electricity; it wets the low melting point metal with which it is in contact.

I find that boric acid not only wets" the low melting point metal, but. in the forms of fuse in which it is in contact with the filling powder with which the cartridge is usually charged it also effectively wets the said powder, and this materially improves the action in respect to the sealing. The term "wets" is employed in the sense in which it is used in connection with surface tension.

I have found that boric acid not only p0 these characteristics, but can efliciently be applied to the fuse metal, and is stable under all the conditions met with in practice provided that care is taken to protect it from moisture.

The most important requirements with which the clearing agent must comply are that it must not be a conductor of electricity, and must not under the reaction which takes place in the fuse, become or form a substance which is a-conductor of electricity. Borax, for example, does not comply with these requirements, and does not therefore fall within my invention.

I have illustrated my invention in the accompanying drawings, in which:-

Flgs. 1 to 3 show the arrangement of a bimetallic fuse as heretofore constructed.

Figs. 4 to 9 show the improvements effected to this type of fuse according to the present invention, and

Figs. 10 to 16 show the invention applied to the now well known type of bi-metallic fuse known as Aeroflex fuse.

Figs. 1'7 and 18 illustrate the application of the invention to a powder-filled fuse in which the fusible element is a metal of low melting point, and

Figs. 19 and show a modification comprising two separate powder-filled fuses employed in combination.

Fig. 21 shows a modification including a connecting. strip.

Fig. 1 is a face view which shows the two copper parts of the fuse,

Fig. 7 illustrates the behaviour of the fuse of Fig. 4 under a short circuit, and

Figs. 8 and 9 which are respectively side and face views to an enlarged scale show the behaviour of the fuse under a moderate excess overload,

Figs. 10 and 10A are views of the two copper strips of which the copper part of the Aeroflex fuse ,is composed. I

Fig. 11 shows these parts in end view ready for assembly and attachment to the capsule of low melting point metal,

Fig. 12 is a face view of the parts in their attached position,

Fig. 13 is a view of the element of Fig. 12 with a coating of boric acid applied,

Fig. 14 is a section on the line XIV, XIV of Fig. 13. v

Fig. 15 is a sectional view to'a larger scale, showing the behaviour of the fuse when it blows upon a moderate overload, and

Fig. 16 is a view similar to Fig. 14 and shows a modification.

Fig. 17 is an elevation, partly in longitudinal section, and shows the arrangement when the boric acid is applied directly to a low melting point metal of the usual type of powder-filled cartridge fuse.

Fig. 18' is a view similar to Fig. 17 and shows a modification.

Fig. 19 is an elevation of a combination of two fuses, one to deal with short circuits, and the other to deal with moderate overloads, and

Fig. 20 is an end view of Fig. 19.

As stated, the preferred form of the invention is that in which the boric acid is applied to the low melting point metal of a powder-filled bimetallic fuse, and this form will therefore be described in the first instance.

Referring first to Figs. 1, 2 and 3 which show a simple form of bi-metallic fuse element:-

The copper strip A has its cross-sectional area locally-reduced at the two parts a3, so that the reduced area is sufficient to carry the currents each side, which may be soldered to the respective ends a2 of the copper strips A. so as to give Fig. 2 is a face view showing the copper parts good contact between the plates and the strip; the plates may be of tin, or lead, or tin-lead alloy, and are so proportioned as to melt when a predetermined overload has been carried for a predetermined time. 6

The element of Fig. 2 would, according to the practice heretofore adopted, be embedded in the powder filling of a cartridge C; the fuse terminal connections (which are omitted in the drawings) are made to the parts al of the fuse l0 element A; on a short circuit the parts a3, as stated, fuse and clear the circuit satisfactorily; but in those cases where the fuse continuously carries current just under the overload at which the fuse should blow at the plates B, the 15 latter are, when the said overload occurs, melted or partly melted but are held up by the filling powder D, (as will be understood from an inspection of Fig. 3) so that the circuit is not broken, under which circumstances sufficient heat 20 is produced to damage the fuse by burning; and to remove this objection the plates B are coated with boric acid.

The process of coating is as follows:-

The boric acid is prepared for use from, for example, the ordinary form of soapy crystal B2O3.3H2O by reducing the H20 content by heating until a plastic adhesive paste is obtained, which is applied over the metal plates B as shown at E in Figs. 4 and 5; the coating is smoothed over and is served with a coat of paint to prevent it from absorbing moisture; the fusible element so constructed is embedded in the filling powder of marble dust or the like D of the cartridge C as shown in Fig. 5. Should the fuse blow under an exceptionally heavy overload, such as a short circuit, the portions a3 fuse instantly, the arc is quickly quenched in the ordinary way by the powder in which the fuse is embedded, and the parts B of the fuse are left 4,0 intact in the powder, as shown in Fig. '7.

Assuming the fuse to be designed to "blow within a prescribed time for an overload of 30% in excess of the normal current, this overload would not fuse the portions a3, but would gradually heat up the low melting point metal plates B, finally melting them; the action of the boric acid under these circumstances is-to ensure that the metal B shall then run. so as to form deflnite transverse fractures across the plates B, and so break the circuit; the boric acid during this action remains a nonconductor, and even when decomposed does not form a product which is a conductor.

Further, as stated, the boric acid not only ensures the free running of the metal of the plates B, but also seals over the fractured ends of the plates B, as shown at bl in Figs. 8 and 9.

In applying the invention to an Aeroflex type of lei-metallic fuse in which the metal of low melting point is made in the form of a capsule B3, the arrangement adopted is shown in Figs. 10 to 15. The capsule is formed from a tube of lead, tin, or lead-tin alloy so shaped that its ends are adapted to be rigidly secured to the extensions a6 formed on the copper strips A, which have portions a3 of reducedarea which, as described, fuse upon short circuit. As before the gap between the two halves of the copper fuse is bridged across in this case by a capsule of the metal of low fusing point, which is designed to melt at prescribed maximum moderate overloads.

A small connecting strip such as a4 shown in Fig. 21, may be left to connect the opposite ends of the copper strips A; this strip is found useful in aligning the finished fuse parts, and does not interfere with the action of the fuse since, when the metal capsule B3 melts, the current causing the melting is wholly carried by and at once fuses the connecting necks or strips a4.

Wit-h an exceptionally heavy overload, such as a short circuit, the parts a3 are instantly fused, leaving the central portion including the capsule intact, as described in connection with Fig. '7.

Two reverse handed copper strips A see Figs.

10 and 10A are preferably employed, and the respective pairs of parts a3 are separated slightly by bending; this permits the filling powder to come into more complete contact with these parts of the copper strip.

The parts ready for connection are shown at Fig. 11; and a fuse of this type assembled, connected and ready to take the coating of boric acid is shown in Fig. 12. The boric acid is applied, as described, a smooth finished surface being obtained, which is painted or varnished to prevent fracture, and the absorption of moisture; and the fuseduly coated so as to completely enclose the capsule is, as shown at Figs. 13 and 14, placed in position in the cartridge C and embedded in the filling powder D.

As before, upon a short circuit the portions 03- (which may be provided with calibrating holes (15) are fused, leaving the central portion intact. When the fuse blows upon a prolonged moderate overload the melting of the capsule is ensured by the presence of the boric acid, and the severed surfaces are sealed by the boric acid as described, and as shown in Fig. 15.

In the type of bi-metallic fuse first described. the capsule is (as shown at F Figs. 14 and 15) preferably filled with boric acid, as well as being coated therewith; the form of the boric employed for the latter purpose being that described above as suitable for filling tubular fuse elements.

In a modification the externally applied boric acid, instead of being applied as a coating as described, may where contact between the boric acid and the filling-powder is not required, be employed as a filling for a small container (of suitable material which is not, and will not in the fusing action, form a conductor) arranged so as to surround the capsule. For example, a container G, Fig. 16, which can be threaded on the fuse element, and the open upper end of which can be closed by a cover gl, may be employed. cl are the fuse terminal contacts.

In those cases in which the fusible element of the enclosed powder-filled fuse is a single metal of low melting point, the boric acid is preferably applied as shown in Fig. 17, in which, as before, C is the fuse cartridge, cl the fuse terminal contacts and D the filling powder; the boric acid E is applied to the central part hl of the fuse H, and is in contact with the filling powder. In a modification, as shown at Fig. 18 the boric acid may, where it is not necessary for it to be in contact with the filling powder, be held in contact with the part M of the fusible element by means of a container G which surrounds the part hi, the arrangement being substantially as described in connection with Fig. 16, but modifled to suit the type of fuse. In some cases the whole of the fuse H may be coated with\the boric acid. 1

In a fuse of the type shown in 17 and 18, the metal of low melting point is best adapted to deal with moderate overloads, and is for this reason fitted in bi-metallic fuses; it is, however,

not best adapted to deal with heavy short circuits, and it is therefore desirable, as shown in Figs. 19 and 20, to fit in series with a powderfilled fuse P having a single fusible element of low melting point with the boric acid coating as 5 cuits, may be used in series with a fuse P of the type illustrated in Figs. 17 and 18.

The quantity of boric acid to be applied to the fuse metal will depend upon the type and rating of the fuse. In a fuse of the type described in Figs. 4 to 9 the coating should be sufficient to 20 enclose the plates B, and in the fuse described in Figs. 12 to 16, to cover the capsule B3 with a thickness of wall of about one sixteenth to one eighth of an inch; in such fuses of amperes capacity the plates B and capsule B3 would be 25 about one inch long and three quarters of an inch wide. In the fuse illustrated in Figs. 12 to 16 the capsule would also be filled with the boric acid. In fuses of the type described in Figs. 17 and 18 the portion hl covered by the boric acid 30 would be about one inch long, and the coating E should have a wall thickness of one sixteenth to one eighth of an inch.

Whilst the invention is particularly applicable to the types of fuse in respect of which it has 35 been described, by way of example, it is applicable to electric fuses generally, to ensure certain and definite clearing under sustained moderate overloads; the details of construction, and mode of application of the boric acid being varied to 40 suit the size and type of fuse to which the invention is applied. 1

Having now fully described my invention I declare that what I claim and desire to secure by Letters Patent is:

1. An improved electric fuse comprising a fusible element having a metal of low-melting point embedded in an arc-quenching powder, and boric acid held in position in contact with the surface of the said metal to cause the metal 50 to run when it is melted by an overload current.

2. An improved electric fuse comprising a fusible element having a metal of low-melting point embedded in an arc-quenching powder, and a coating of boric acid in the plastic state on said metal and embedded in the filling powder for causing the metal to run when it is melted by an overload current.

3. An improved electric fuse comprising a fusible element having a metal of low-melting point embedded in an arc-quenching powder, said metal being of hollow form, and boric acid charging the interior of the metal and also applied to the exterior thereof and embedded in the filling powder for causing the metal to run when it is'melted by an overload current.

4. An improved electric fuse comprising a fusible element having a metal of high fusing point and a metal of low-melting point connected in series, both of said metals being embedded in an arc quenching powder, and boric acid applied to said metal of low melting point and held in position in contact with the surface of said metal I when it is melted by a moderate overload current.

5. An improved electric fuse comprising a fusible element having a metal of high fusing point and a metal of low-melting point connected in series, both of said metals being embedded in an arc-quenching powder, the metal of low melting point being of hollow form, and boric acid charging the interior of the hollow metal and on the exterior thereof and held in contact therewith and embedded in the filling powder for causing the metal of low melting point to run when it is melted by'a moderate overload current.

6. In an electric fuse in combination; an enclosing casing provided with electric contacts; an arc-quenching powder with which the casing is charged; a fusible element embedded in the arc-quenching powder, and the ends of which are connected to the said contacts; and boricacid applied to the fusible element so as to be in contact with the surface of the latter and the filling powder.

7. In an electric fuse in combination; an enclosing casing provided with electric contacts; an arc-quenching powder with which the casing is charged; a fusible element embedded in the arc-quenching .powder, the ends of which are connected to the said contacts, said fusible element having a hollow portion; a charge of boric acid located within the said hollow portion, and boric acid held in contact with the exterior surface of the said hollow portion and the filling powder.

8. In an electric fuse in combination; an enclosing casing provided with electric contacts; an arc-quenching powder with which the casing is charged; a bi-metallic fusible element embedded in the arc-quenching powder, and the ends of which element are connected to the end contacts, said fusible element consisting of two end portions of a metal of comparatively high fusing point, each of the said portions being reduced in cross-section so that fusing takes I place at the said reduced cross-section upon the occurrence of an exceptionally high overload such as a short-circuit, the adjacent ends of the said high fusing point metal being connected by a metal having a low melting point adapted to be melted when the fuse carries a current substantially equal to the minimum overload at which it is designed to fuse; and boric acid applied to the metal of low melting point and held in position in contact with the said low melting point metal and the filling powder. 10

9. In an electric fuse in combination; an enclosing casing provided with electric contacts; an arc-quenching powder with which the case is charged; a bi-metallic fusible element embedded in the arc-quenching powder, and the 15 ends of which element are connected to the end contacts, said fusible element consisting of end portions of a metal of comparatively high fusing point, each of the said portions being reduced in cross-section so that fusing takes place at the 20 said reduced cross-section upon the occurrence of an exceptionally high overload such as a short circuit, the adjacent ends of the metal of high melting point being connected by a hollow portion formed by a metal of low melting point; 25 boric acid applied to the exterior of the said hollow portion and held in contact with the surface thereof and the filling powder; and a charge of boric acid located within the said hollow portion.

10. In a fuse constructed in accordance with claim 1, a containing vessel attached to the fusible element, the said vessel being adapted to contain the boric acid in the form of a powder and to maintain the said acid in contact with 35 the fusible element.

11. In an electrical fuse constructed in accordance with claim 4, a powder filled casing for the fusible element of high melting point, and a powder filled casing for the hollow fusible ele- 4n ment of low melting point; electrical contacts on said casings by means of which contacts the two fusible elements are connected in series in the circuit to be protected.

VERNON HOPE. 

